Multiple monitors wallpaper handled as only one big monitor.

That upstream bug seems to suggest there's no such problem in openbox?

In any case, this ideally is a WM issue, reading that upstream report.

lxde also uses openbox and you can put different wallpapers for each monitor. When you change the wallpaper it only changes the one in which you have done the right click with the mouse.

Apparently this issue also happens in gnome (I haven't used gnome since 2000 and never in 2 monitor environment). In a youtube video they recommend to use hydra-paper https://github.com/GabMus/HydraPaper/ to "create" the wallpaper merging the 2 images in one and use this as wallpaper.

So, maybe the best/simple solution is to include a good wallpaper manager in lubuntu next.

Here is the video https://www.youtube.com/watch?v=3svfVEiOCBU&t=329s

I actually apparently didn't read that bug correctly. The issue is not the window manager at all. It's pcmanfm-qt. Just as with LXDE, the desktop (read: wallpapers) are managed by pcmanfm (see the --desktop switch, which we run by default).

That said, this is an upstream issue that will require a non-trivial fix, especially as they are trying to be agnostic in relation to components, meaning not just the window manager but also the display server so it's a bit of a rabbit hole:
https://github.com/lxqt/lxqt/issues/1175#issuecomment-253859198

Too bad HydraPaper is GTK. We probably need something lighter. feh can certainly handle this but it's a little cludgy (CLI only) but maybe we can make a little script. There's a bunch of other options to explore, too:
https://wiki.archlinux.org/index.php/List_of_applications#Wallpaper_setters

Someone who readily uses multiple monitors needs to test this stuff and report what works. If we do get a good solution, I'd be inclined to include it with the expectation that we'll eventually remove it once upstream figures things out.

I tried all the wallpaper setter that are in ubuntu repositories: Feh, Nitrogen and Variety. Nitrogen and Feh are able to manage the 2 monitors w/o problems and variety runs in backround continuously changing wallpaper, and I couldn't find out how to manage multiple monitors.
The downside is that all of them required to kill "pcmanfm-qt --desktop".

Keep looking for a good workaround, Hans. Perhaps the cron job that checks to see if the wallpaper has changed and then uses imagemagick to make it the right size for the "one monitor" will solve the issue, but it's not a good permanent fix. Looking outside of pcmanfm-qt has ultimately led us right back to it. Because of that, I'm going to set the importance to wishlist.

I just left a comment at the upstream bug in hopes of spurning development. The long story short is that there's no consistency among window managers and so detection is necessary, but they are reluctant to make a dependency on an X11 component to do this (cuz Wayland). I suggested we simply make a non-default compile time flag to turn this on. Hopefully that will be enough to encourage them to do the work for the immediate need. I can't imagine the code they write won't be in some way useful when Wayland comes around.

I forgot adding the current workaround which is here:
https://code.launchpad.net/~hmollercl/stitchwp/+git/stitchwp

Solved with LXQt 0.15 in lubuntu 20.10